The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a trench and a method for fabricating a semiconductor device using the same.
Generally, a recess gate process, a shallow trench isolation (STI) process of a flash memory device, or the like, essentially employs a process of forming a trench in a substrate by partially etching the substrate to a predetermined depth. In the forming process of a trench, it is desirable to form the trench such that its sidewalls have a vertical profile for various reasons. For instance, a process of forming a trench for a recess gate will be described hereinafter.
A recess gate process can increase a channel length of a cell transistor to thereby improve refresh characteristics of a device by forming a gate pattern of which an upper portion protrudes from the surface of the substrate while filling a trench formed by etching a predetermined portion of an active region of the substrate. To improve properties of semiconductor devices, more advanced technology has recently been announced in which an upper portion of a gate has a vertical profile and a lower portion has a spherical profile, which is called a bulb type recess gate process. In the bulb type recess gate, the upper portion of the gate having the vertical profile is referred to as a neck pattern and the lower portion of the gate having the spherical profile connected to the neck pattern is referred to as a bulb pattern.
It is required that the sidewall of the trench in the recess gate process, or the sidewall of the neck pattern in the bulb recess gate process, should have a vertical profile so as to prevent a seam in a conductive layer, e.g., a polysilicon layer, for a gate filled into the trench.
In a recess gate process currently used, however, an etch barrier layer used in etching the substrate to form a trench is an oxide layer, and thus a bowing phenomenon takes place at a sidewall profile of the trench because small amount of byproduct results in the decrease in passivation effect of the trench sidewall (see A of FIG. 1). To improve the bowing phenomenon, the etch process may be performed using an etch gas obtained by adding hydrogen (H2) gas into nitrogen (N2)/chlorine (Cl2) gas. However, it is not preferable to use the H2 gas because it is too dangerous. Further, a field oxide layer may be lost during the etch process of forming the trench for a recess gate (see B of FIG. 1).
In addition to the aforementioned recess gate process, the trench forming process for a semiconductor memory device encounters a great challenge to achieve the effective passivation of the trench sidewall and thus to prevent the bowing phenomenon of the trench sidewall profile during all the processes of forming a trench by selectively etching a substrate, for example, in the case where the trench sidewall for device isolation must be formed vertically in an STI process of a flash memory device.